Question

Dari barisan aritmetika diketahui u3 = 18 dan u7 = 38. jumlah 24 suku pertama adalah?

Answer 1

U3 = 18 berarti a + 2b = 18 (1)
u7 = 38 berarti a + 6b = 38 (2)
                     ------------------- (dikurang)
                        -4b = -20
                           b = 5

a + 10 = 18 (1)
a = 8

Sn = n/2(a+un)
u24= 8 + 23x5 = 8 + 115 = 123

S24 = 12 (8+123)
S24 = 12 (131)
S24 = 1572

Answer 2

  u7 = 38
  u3 = 18      
  u4 = 20
 b   = 20/4
      = 5

u3 = 18
u2 = 13
u1 = 8

s24 = n/2 (2a + (n-1)b)
      = 24/2 (2.8) + (24-1)5)
      = 12 (16 + (23.5)
      = 12 (16 + 115)
      = 12 (131)
      = 1572